1. Field of the Invention
This invention relates to a nonvolatile semiconductor memory device capable of storing, for example, two bits or more of data.
2. Description of the Related Art
A nonvolatile semiconductor memory device capable of storing multivalued data, such as a NAND flash memory using EEPROM, has been proposed (for example, refer to Jpn. Pat. Appln. KOKAI Publication No. 2000-195280).
In a NAND flash memory, all of or half of a plurality of cells arranged in the row direction are written into or read from simultaneously. The NAND cells constituting each NAND flash memory are connected via bit lines to a write and read latch circuit.
In a NAND flash memory which stores multivalued data, a threshold voltage according to the writing data is set in a memory cell, which enables a plurality of bits of data to be stored in the memory cell. To suppress the threshold voltage distribution, writing is done at a threshold voltage lower than the original threshold voltage in a first write operation. Then, in a second write operation, writing is done to the original threshold voltage. When data is written by this method, the writing data is not left in the data storage circuit after the first write operation. For this reason, after the first write operation, the writing data is read out in a read operation and then determined. However, the data written in the first write operation cannot be read accurately because it is lower than the original threshold voltage. To overcome this problem, it is necessary to store three bits of data, including the first writing data, the second writing data, and the data written in a lower page (a page preceding the page now being written into). A storage circuit for storing those data items is composed of, for example, two CMOS latch circuits and a precharge transistor for holding the data read from the memory cell in verifying the data bit by bit and forcing the data to be set to data “1”. As a result, the storage circuit has the demerit of occupying a larger area. Therefore, there has been a need for a nonvolatile semiconductor memory device which is capable of eliminating a storage circuit for storing the data necessary for writing and of writing multivalued data with a simpler circuit configuration.